The present invention relates to a method of making an aerofoil blisk employing linear friction welding.
FIG. 1 shows an aerofoil blisk 10 which includes a plurality of blades 12 attached to a disc 14 so as to extend radially outwardly therefrom. The blades are typically of titanium, nickel or steel and are attached to the disc by linear friction welding. Blisks may be used in aero engines, both in the compressor and turbine, and can be advantageous over conventionally bladed discs.
It is known to join the blades 12 to the disc 14 by linear friction welding, this being a process whereby the disc 14 is held stationary while a blade member (comprising the as yet unmachined/unfinished blade) is oscillated tangentially against the disc 14 under a load applied in the radial direction of the blade towards the disc. The heat generated by the oscillation together with the radially inward load results in a weld between the disc 14 and the blade member, with weld material being extruded from both sides of the joint.
The blade is thereby joined to the disc.
Excess material is subsequently machined away from the blade member, to result in a blade of the desired shape.
The radially inner base region of the blade member which contacts the disc 14 for joining thereto by linear friction welding is referred to as a stub. FIG. 2 shows schematically the cross-sectional shape of a prior art blade stub 16. The section is transverse to the length of the blade member, i.e. tangentially to the disc when the blade member is attached thereto. The weld oscillation direction is tangential, as indicated by the arrow A.
The stub 16 of the blade member includes a leading edge 18 and a trailing edge 20, each being smoothly curved. The stub further includes a suction side 22 and a pressure side 24, each of which curves smoothly between the leading and trailing edges 18 and 20, on its respective side of the blade member.
The curvature of the pressure side 24 is somewhat less than that of the suction side 22 and the stub 16 has a maximum width measured in the weld oscillation direction which is significantly greater in its central region than at its leading edge 18 or trailing edge 20. The arrows X indicate the leading edge width of the blade, the arrows Y the trailing edge width of the blade and the arrows Z the maximum weld width. The ratio of the maximum weld width to the minimum weld width is more than 2. This ratio is referred to as the taper ratio.
Having a relatively high taper ratio causes problems with the friction welding process. As the linear friction welding process takes place, “flash” material is pushed out to the sides of the weld, i.e. it is burnt-off. As the material is pushed out, the radially inward pressure (the forge pressure) forces the blade member in the radially inward direction. The burn-off rate of material is higher in the regions where the stub is relatively narrow in the weld oscillation direction. In these regions, the blade member does not move in the radially inward direction fast enough to keep up with the rate of burning off of material. This is because the material in the wider regions of the blade stub prevents such radially inward movement (the burn-off rate being lower in these regions). This can result in the recirculation of flash in the narrower regions and even in voids in the weld. Recirculation is damaging to the weld quality/integrity. To address these problems, EP 1495829 A discloses an approach for making an aerofoil blisk by linear friction welding in which the stub has a taper ratio of less than 2.
However, the approach of EP 1495829 A does not account for variation in energy input into the weld at different positions across the stub (e.g. at leading edge, mid-chord and trailing edge positions) caused by differential amplitude of oscillation or differential forge pressure, the differentials being caused by elastic deflections in the components and/or the tooling under the high forge and in-plane loads.